This invention relates to the field of high-speed mechanical impact printers, and in particular, it relates to an electromagnetically actuated hammer mechanism used to strike the printing elements in such printers.
The advent of high speed data processing and word processing equipment has brought about a need for high-speed printers capable of producing high quality printing. Perhaps the most popular type of printer in such applications is that which uses the so-called "daisy wheel". The daisy wheel basically consists of a disc having a plurality of flexible fingers or petals around its periphery, each of which bears a particular print character. In operation, a daisy wheel is rotated until the selected character is in position for printing, at which time a clapper is electromagnetically driven against a hammer, which in turn drives the print element against the paper to be printed. A typical example of such a printer is disclosed in U.S. Pat. No. 3,954,163 to Gabor.
In such printers it is necessary to have a significant clearance space between the hammer and the print wheel or daisy wheel when the hammer is withdrawn so that the print wheel may rotate freely without danger of striking the hammer and damaging the relatively fragile spokes or petals of the daisy wheel. Accordingly, the hammer must have a relatively long stroke so that it can be withdrawn a significant distance away from the print wheel as the print wheel rotates, while being capable of driving the printing element against the paper to be printed when the print wheel has stopped. The obvious approach to achieving such a long hammer stroke would be to propell the hammer by a strong magnetic field provided by a relatively large solenoid. However, in such printers, in order to achieve high speed, it has been found necessary to move the hammer assembly and print wheel across the platen by means of a servo-driven motor. Accordingly, it is desirable to keep the hammer assembly as light in weight as possible, thereby effectively precluding the use of the large, heavy solenoid which would be necessary to effect the required length of hammer stroke. Consequently, in order to make use of relatively small, lightweight electromagnetic coils, the typical prior art daisy wheel printer uses the coils to drive a relatively long clapper arm, much like that used in an electric doorbell. The end of the clapper arm travels in a relatively wide arch to strike the hammer, thereby allowing the hammer to have the necessary long stroke while using coils which are relatively small and lightweight.
Such a hammer assembly has several drawbacks. The first, of course, is complexity, which affects reliability. More importantly, the driving of the hammer indirectly through a second driven element, i.e., the clapper, limits the ability to control the impact force of the hammer against the print element.